There are multiple ways you can obtain permanent residency (PR) in Australia. Pathways to PR will generally fall into 3 categories:
1. Skilled Independent
You could obtain a Skilled Independent visa directly.
You will have to pass a points test that will rank you based on occupation, education level, age, English language ability, years of work experience, etc.
To know more about the points-based system for Skilled Independent migration, refer to SkillSelect.
Part 3 of this document describes the points system in detail. The current points pass mark, required to be able to apply, is 60. Use this calculator to estimate your points. After you have established you have 60 points, you should submit an Expression of Interest. Every month the department invites people to apply, based on a ranking of those who submitted an EoI.
2. Employer sponsorship
You could try a sponsorship by an Australian employer under the Employer Nomination Scheme.
You will need to get an employer that's willing to sponsor you for a permanent visa. Not many employers directly sponsor employees for a PR. It's far more common to be first sponsored for a temporary work visa.
3. Temporary to permanent pathway
You can obtain a temporary visa (work or study) first and applying for PR later.
This option is useful if you are not directly eligible for the other 2 options. Spending time working or studying in Australia increases your chances of obtaining a PR. Having a degree from an Australian institution or Australian work experience will give you more points.
These 3 options are not necessarily mutually exclusive. You could try to obtain a temporary visa while you sort out the application for a skilled independent visa, or even after applying, while your application is being processed. However, both options will take quite a lot of effort and especially, money. I would first check for eligibility under SkillSelect, and submit an Expression of Interest.
Skilled Occupation List
The skills that are in high demand in Australia are listed in the Skilled Occupation List (SOL). Several software development professions are in the list.
In addition to satisfying the required work experience of the points system, you will need to satisfy the assessment of a professional body, which might have its own work experience requirement. In your case, the Australian Computer Society (ACS) will do the assessment. In all cases, work experience needs to be proven. Any experience you plan to claim, e.g. your freelance work or job, will need to be substantiated by documentation. These may include payslips, employer letter, receipts, tax documents. These things have to be done before you submit an EoI.